1. Field of the Invention
The present invention relates to a strain gage for detecting load added to a structural member and strain thus caused in the structural member when the structural member is elastically deformed by the load added.
2. Description of the Related Art
When load is added to a structural member and strain is thus caused in it by the load added, this load or strain is detected using various types of strain gages. One of these strain gages is of the metal resistance type, which has a metal resistance.
When load added or strain thus caused by the load is to be detected, the strain gage of this type is bonded to the structural member. The resistance value of the metal resistance changes in response to the load added and detection signal which represents this change of the resistance value is outputted. The load is thus measured and the strain is obtained from the load measured because the strain is in proportional to the load. The strain gage of this type is more frequently used as compared with those of the other types because its handling is relatively easier and its reliability is relatively higher.
However, the strain gage of this type is relatively lower in its detecting sensitivity. Two measures are sometimes employed to enhance its detecting sensitivity.
When the rigidity of the structural member is relatively high, the first measure employed is to increase the gain of detection signal. When the gain of detection signal is increased like this, however, SN ratio of detection signal becomes low.
When drive force generated by the arm of a robot which is the structural member is to be measured or when load added to the arm of the robot is to be measured, the second measure employed is to make relatively low the rigidity of that portion of the arm to which the strain gage is bonded. That portion of the arm to which the strain gage is bonded can be thus elastically deformed to a great extent to change detection signal. The detecting sensitivity of the strain gage can be enhanced accordingly. When the rigidity of the arm is made low, however, its response capacity is worsened.
This leads to a conclusion that the two measures which are employed to enhance the detecting sensitivity of the strain gage are not preferable.
The strain gage whose detecting sensitivity is relatively higher is of the semiconductor type. In the case of this strain gage of the semiconductor type, however, its detection characteristic changes as its temperature changes. Unless its temperature is kept in a certain range, therefore, detection signal outputted from it is not correct. This strain gage is low in reliability because of its possibility of creating detection errors. In addition, its handling is troublesome because its temperature must be controlled. Furthermore, it is difficult to perform complementary temperature compensation using two through four gages, since characteristic change rate caused by change in temperature is different from gage to gage. This is the reason why the strain gage of this type is hardly used.
There is thus needed a strain gage which has higher detecting sensitivity and reliability and which can be handled with more easiness.